arch: riscv: fatal: make `cause_str` reusable

Rename `cause_str` to `z_riscv_mcause_str` and make it non-static,
so that it can be used in user-implemented `k_sys_fatal_error_handler`.

Also, this function should return a constant string, so add `const`
to it.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>
Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>